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  1. #include <mpi.h>
  2. #include <stdio.h>
  3.  
  4. int main(int argc, char** argv) {
  5.     MPI_Init(&argc, &argv);
  6.  
  7.     int rank, size;
  8.     MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  9.     MPI_Comm_size(MPI_COMM_WORLD, &size);
  10.  
  11.     // Assumptions based on problem: 7 ranks, each with part of the data
  12.     const int rows_per_rank = 25; // Each rank handles 25 rows
  13.     const int total_columns = 100; // Total columns in the matrix
  14.     float apart[rows_per_rank][total_columns]; // Partial matrix on each rank
  15.     float b[total_columns]; // Vector for multiplication
  16.     float cpart[rows_per_rank]; // Partial result on each rank
  17.     float ctotal[rows_per_rank * size]; // Final result (gathered from all ranks)
  18.  
  19.     // Initialize the matrix 'apart' and vector 'b' with some values
  20.     // For example, here we fill them with 1s to simplify.
  21.     for (int i = 0; i < rows_per_rank; i++) {
  22.         for (int k = 0; k < total_columns; k++) {
  23.             apart[i][k] = 1.0f; // Example value, could be anything
  24.         }
  25.     }
  26.  
  27.     // Initialize vector 'b' with 1s (again, as an example)
  28.     for (int k = 0; k < total_columns; k++) {
  29.         b[k] = 1.0f;
  30.     }
  31.  
  32.     // Each rank calculates its part of the matrix-vector product
  33.     for (int i = 0; i < rows_per_rank; i++) {
  34.         cpart[i] = 0.0f;
  35.         for (int k = 0; k < total_columns; k++) {
  36.             cpart[i] += apart[i][k] * b[k];
  37.         }
  38.     }
  39.  
  40.     // Use MPI_Allgather to gather the partial results from all ranks into 'ctotal'
  41.     MPI_Allgather(cpart, rows_per_rank, MPI_FLOAT, 
  42.                   ctotal, rows_per_rank, MPI_FLOAT, 
  43.                   MPI_COMM_WORLD);
  44.  
  45.  
  46.     // For demonstration, let's print the results on rank 0
  47.     if (rank == 0) {
  48.         printf("Final result vector (gathered from all ranks):\n");
  49.         for (int i = 0; i < rows_per_rank * size; i++) {
  50.             printf("%f ", ctotal[i]);
  51.         }
  52.         printf("\n");
  53.     }
  54.  
  55.     MPI_Finalize();
  56.     return 0;
  57. }
  58.  
Success #stdin #stdout #stderr 0.26s 40536KB
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Standard input is empty
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Standard output is empty
stderr
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Error: unexpected symbol in "void MPI_MyAllgather"
Execution halted
https://ideone.com/T17CaQ
language:
R (R 3.5.2)
created:
4 days ago
visibility:
public

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